Hemiboreal forests? CO2 fluxes response to the European 2018 heatwave

In summer 2018, Europe experienced a heatwave that impacted the forest carbon cycle. We assessed the influence of elevated temperatures on the carbon exchange of three forest stands of different types (coniferous, deciduous, and conifer-broadleaved) and a clear-cut area located in the geographical vicinity within a hemiboreal zone. The carbon fluxes were calculated using the eddy-covariance technique. Over the period of anomaly high temperatures, the NEE of a deciduous (alder) forest was similar to the previous year due to the reduction of both GPP and ER of the same magnitude. Warm days in spring and autumn allowed for the GPP offset, and the total NEE of vegetation season 2018 was more negative than of 2017. Coniferous (pine and spruce) forest carbon uptake decreased during the temperature anomaly as a result of GPP reduction being higher than the decrease in ER. Spring warming had almost no effect on the carbon exchange, while ER and GPP increased in autumn warm days. As a result, annual ER was higher in 2018 than in 2017, and the NEE was smaller. The coniferous forest was most resistant to the heatwave, most probably because of its adaption to low soil water content. Contrary to our expectations, a mixed conifer-broadleaved forest (pine, spruce, birch, clear-cuts) was more affected by the heatwave than a pine-dominated forest. It switched from net carbon sink during the vegetation season of 2017 to net carbon source in 2018. Almost three times higher amount of carbon was released in 2018 as a result of GPP reduction. The clear-cut area carbon exchange was the most vulnerable to the heatwave influence out of all study sites. A more prominent reduction of GPP resulted in smaller NEE. Warm days in spring and autumn did not offset the temperature anomaly impact, as was observed for the forested ecosystems.